1. Field of the Invention
The present invention relates to a process for preparing a pentenoic ester, and more particularly to a process for preparing a pentenoic ester by using a 3- and 4-formylvaleric ester as the starting material in the presence of a specific mixed oxide-supported noble metal as the catalyst.
2. Description of the Prior Art
The conventional process for synthesizing caprolactam involves first reacting butadiene, carbon monoxide, and an alcohol to form a pentenoic acid, which is hydroformylated to form a formylvaleric ester. Then, the formylvaleric ester is reacted to form a 6-aminocaproate by amination, which is then cyclized to form caprolactam.
In the above process, when the pentenoic ester is hydroformylated, three kinds of isomers, including 3-, 4-, and 5-formylvaleric esters, are formed. However, only 5-formylvaleric esters can be reacted to form 6-aminocaproates, and thus 15-20% of the formations are undesirable byproducts, including 3- and 4-formylvaleric esters. Many attempts have been proposed to turn 3- and 4-formylvaleric esters back to pentenoic esters via dehydrocarbonylation, which can be used to prepare caprolactam, thus saving the cost of the raw material. 3- and 4-formylvaleric esters can be reacted to three kinds of isomers including 2-, 3-, and 4-pentenoic esters via dehydrocarbonylation. 3- and 4-pentenoic esters can be effectively reacted to 5-formylvaleric esters via hydroformylation, which can further be reacted to caprolactam. However, 2-pentenoic esters are very difficult to hydroformylate and are reacted to peroxides with oxygen. The result is that the hydroformylation catalyst loses its activity. Therefore, it is very important to decrease the selectivity of 2-pentenoic esters and increase the selectivity of 3- and 4-pentenoic esters.
Theoretically, when an aldehyde is reacted to an olefin, a Group VIII noble metal such as Pd (palladium), Pt (platinum), and Rh (rhodium) can be used as the catalyst.
J. Amer. Chem. Soc. 90 (1968), 94-98 discloses that n-decanal (an aldehyde) can be converted in the presence of palladium or platinum supported on active carbon to give olefins via dehydrocarbonylation. In addition, J. Amer. Chem. Soc. 90 (1968), 99-107 discloses that n-heptanal can be converted in the presence of rhodium complexes to give hexane by dehydrocarbonylation.
U.S. Pat. No. 4,517,400 describes that when palladium, platinum, or rhodium is present, the straight-chain aldehydes can be cleaved to give olefins, while the branched aldehydes show virtually no reaction.
German Pat. No. 1,917,244 describes a process for dehydrocarbonylating isobutyraldehyde using rhodium-containing alumina as a catalyst at 280xc2x0 C., to 330xc2x0 C.
European Patent Application No. 81,090 describes that formylvaleric esters, if heated to 150xc2x0 C. to 600xc2x0 C., undergo cyclization to 3,4-dihydro-2-pyrones.
In U.S. Pat. Nos. 4,879,405 and 4,879,406, 3- and 4-formylvaleric esters can be converted in the presence of a Group VIII noble metal as a catalyst at 50xc2x0 C. to 400xc2x0 C. to give pentenoic esters via dehydrocarbonylation. However, the total selectivity of the 2-pentenoic ester and valeric ester is higher than 20%, and the total yield of the 3- and 4-pentenoic esters is lower than 80%.
Therefore, the object of the present invention is to solve the above-mentioned problems and to provide a process for preparing a pentenoic ester by converting 3- and 4-formylvaleric esters. The yield of the pentenoic ester is higher than 85%, the selectivity of the 3- and 4-pentenoic esters is higher than 85%, and the selectivity of the 2-pentenoic ester can be greatly decreased.
To achieve the above-mentioned object, the process for preparing a pentenoic ester of the present invention includes heating a 3-, 4-formylvaleric ester or mixtures thereof at 50xc2x0 C. to 400xc2x0 C. in the presence of a supported noble metal catalyst.
One aspect of the present invention is that the noble metal catalyst is supported on a mixed oxide (M1)a(M2)b(M3)cPdAleSiOx, wherein M1 is an alkali metal, M2 is an alkaline earth metal, M3 is a Group IVB metal, a=0.5xcx9c1.5, b=0.2xcx9c0.8, c=0.2xcx9c0.8, d=2xcx9c8, e=3xcx9c10, and x is the stoichiometric value.
In the above-mentioned mixed oxide, M1 can be Li, Na, or K, M2 can be Be, Mg, or Ca, M3 can be Ti, Zr, or Hf. Preferably, a=0.7-1.3, b=0.4-0.7, c=0.4-0.7, d=4-7, and e=4-8.
Preferably, the noble metal catalyst used in the present invention contains a Group VIIIB metal, such as Pd (palladium), Pt (platinum), Rh (rhodium) or Ru (ruthenium).
According to the present invention, the specific supported noble metal catalyst includes the noble metal catalyst itself and the mixed oxide (M1)a(M2)b(M3)cPdAleSiOx as the support, which has two functions of dehydrocarbonylation and isomerization. In the presence of such a supported catalyst, 3- and 4-formylvaleric esters can undergo dehydrocarbonylation on the noble metal to give 2-, 3-, and 4-pentenoic esters. Then, the 2-pentenoic ester can be converted via isomerization on the mixed oxide support into 3- and 4-pentenoic esters. In this manner, the selectivity of the 2-pentenoic ester can be decreased. In addition, cyclization to 3,4-dihydro-2-pyrones from formylvaleric esters can be inhibited by means of the mixed oxide. Therefore, the mixed oxide can serve as a catalyst and has the properties of acid and base, which belongs to a solid acid/base catalyst.
The noble metal catalyst can be supported on the (M1)a(M2)b(M3)cPdAleSiOx, mixed oxide by ionic exchange, impregnation, or mechanical mixing. The noble metal catalyst to be supported can be in an amount of 0.01 to 10 wt %, preferably 0.1 to 5 wt %, based on the total weight of the noble metal catalyst and the mixed oxide support.
According to the present invention, the starting materials can be a 3-formylvaleric ester, 4-formylvaleric ester, or mixtures thereof. The starting materials are heated to a temperature of 50xc2x0 C. to 400xc2x0 C., preferably to 100xc2x0 C. to 250xc2x0 C., in the presence of the above-mentioned supported noble metal catalyst. In this way, the starting materials can be converted to a pentenoic ester via dehydrocarbonylation. The 3- and 4-formylvaleric esters can be independently an ester of alkyl, preferably an ester of alkyl having from 1 to 3 carbon atoms. Examples of alkyl are methyl, ethyl, propyl and isopropyl. Representative examples of the 3- and 4-formylvaleric esters include methyl 3-formylvalerate and methyl 4-formylvalerate.
The above-mentioned dehydrocarbonylation is preferably conducted in the presence of an oxygen-containing gas, such as air or oxygen. For example, 10-40% (in volume) of oxygen, which is diluted with an inert gas, can be introduced into the reactants. The inert gas can be nitrogen, carbon dioxide, helium, or argon.
Moreover, in order to minimize excess oxidation and byproduct generation, 0.01-10 wt % of water, based on the total weight of the formylvaleric esters, is preferably added to the starting materials. In addition, in order to avoid hydrolyzation of the formylvaleric esters and the pentenoic esters, alcohols, such as methanol, ethanol, butanol, or cyclohexanol, are preferably added to the starting materials. The alcohol is preferably in an amount of 30-90 wt % based on the total weight of the formylvaleric esters.
The liquid hour space velocity (LHSV) of the formylvaleric esters can be in a range of 0.1 to 10 hrxe2x88x921, preferably 0.5 to 5 hrxe2x88x921.
The conversion and selectivity mentioned in the present invention are calculated according to the following formulae, in which FV indicates formylvaleric ester and PTE indicates pentenoic ester:             conversion      ⁢              xe2x80x83            ⁢              (                  mole          ⁢          %                )              =                                        FV            ⁢                          xe2x80x83                        ⁢            weight            ⁢                          xe2x80x83                        ⁢            in            ⁢                          xe2x80x83                        ⁢            the            ⁢                          xe2x80x83                        ⁢            feedstock                    -                      FV            ⁢                          xe2x80x83                        ⁢            weight            ⁢                          xe2x80x83                        ⁢            in            ⁢                          xe2x80x83                        ⁢            the            ⁢                          xe2x80x83                        ⁢            product                                    FV          ⁢                      xe2x80x83                    ⁢          weight          ⁢                      xe2x80x83                    ⁢          in          ⁢                      xe2x80x83                    ⁢          the          ⁢                      xe2x80x83                    ⁢          feedstock                    xc3x97      100      ⁢      %                  selectivity      ⁢              xe2x80x83            ⁢              (                  mole          ⁢          %                )              =                                                                      PTE                ⁢                                  xe2x80x83                                ⁢                weight                ⁢                                  xe2x80x83                                ⁢                in                ⁢                                  xe2x80x83                                ⁢                the                ⁢                                  xe2x80x83                                ⁢                product                xc3x97                                                                                        molecular                ⁢                                  xe2x80x83                                ⁢                weight                ⁢                                  xe2x80x83                                ⁢                of                ⁢                                  xe2x80x83                                ⁢                                  FV                  /                  molecular                                ⁢                                  xe2x80x83                                ⁢                weight                ⁢                                  xe2x80x83                                ⁢                of                ⁢                                  xe2x80x83                                ⁢                PTE                                                                          mole            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            FV            ⁢                          xe2x80x83                        ⁢            in            ⁢                          xe2x80x83                        ⁢            the            ⁢                          xe2x80x83                        ⁢            feedstock                    -                      mole            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            FV            ⁢                          xe2x80x83                        ⁢            in            ⁢                          xe2x80x83                        ⁢            the            ⁢                          xe2x80x83                        ⁢            product                              xc3x97      100      ⁢      %                  yield      ⁢              xe2x80x83            ⁢              (                  mole          ⁢          %                )              =                                                                      PTE                ⁢                                  xe2x80x83                                ⁢                weight                ⁢                                  xe2x80x83                                ⁢                in                ⁢                                  xe2x80x83                                ⁢                the                ⁢                                  xe2x80x83                                ⁢                product                xc3x97                                                                                        molecular                ⁢                                  xe2x80x83                                ⁢                weight                ⁢                                  xe2x80x83                                ⁢                o                ⁢                                  xe2x80x83                                ⁢                                  FV                  /                  moleculat                                ⁢                                  xe2x80x83                                ⁢                weight                ⁢                                  xe2x80x83                                ⁢                of                ⁢                                  xe2x80x83                                ⁢                PTE                                                              mole          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          FV          ⁢                      xe2x80x83                    ⁢          in          ⁢                      xe2x80x83                    ⁢          the          ⁢                      xe2x80x83                    ⁢          feedstock                    xc3x97      100      ⁢      %      